The present invention relates to aliasing of velocity information. In particular, anti-aliasing of phase-based ultrasound velocity estimates is automatically performed.
Ultrasound velocities are estimated within a range of velocities. The available range of velocities is dictated by the pulse repetition frequency used in pulsed transmissions of ultrasound energy to sample fluids or tissues. Velocities are estimated in response to multiple pulses. Decreasing the pulse repetition frequency (PRF) can increase the time required to generate an image of velocity information and can give under sampled, and hence erroneous velocity information. Conversely, increasing the PRF (for a fixed number of samples) will decrease the precision of velocity estimates that are not aliased, because of quantization effects. Accordingly, users typically adjust the pulse repetition frequency to be as low as possible without aliasing. Aliasing occurs where an actual velocity is beyond the Nyquist sampling range established by the pulse repetition frequency. As a result of aliasing, the velocity data is assigned an incorrect velocity value. The velocity information contains uncertainties of multiples of a specific velocity. If aliasing occurs during imaging, the physician may have to reacquire the velocity images.
Velocity imaging is used for strain rate calculations. A strain rate is calculated over one or more heart cycles, resulting in drastic differences of the maximum velocity as a function of time within the heart cycle. To avoid velocity aliasing at any point during the heart cycle, the pulse repetition frequency is set to include a maximum expected velocity within the heart cycle. The user adjusts the velocity scale and may occasionally underestimate peak velocities. A velocity scale that is too low may introduce errors into the velocity estimates because of aliasing. If aliasing does occur, an error is introduced into any strain rate calculation. Since image acquisition time is limited during a stress echo exam due to the dynamic effect of exercise or drugs during the stress cycle exam, a conservative or high velocity scale is selected by the user. The result is an unnecessarily high quantization error, which provides sub-optimal information. It has been publicly suggested that the narrow range of velocities within a short distance over tissue for a strain rate calculation should enable the strain rate to be computed even in the presence of velocity aliasing. However, no further details were provided.